1. Field of the Invention
The invention relates generally to the field of detection and characterization of particles in concentrated liquid systems, such as slurries and suspensions.
2. Background Information
Liquid systems with high particulate concentrations are widely used in industry. Examples of such systems are slurries used in Chemical Mechanical Planarization (CMP) processes for the semiconductor industry and emulsions used in the pharmaceutical industry.
Slurry systems used for CMP can have a complicated chemical and colloidal composition. Regarding the chemical composition, a slurry can be either basic (relatively high pH) or acidic (relatively low pH). Keeping the pH value in a predetermined range is often desired for slurries because the pH value often determines the stability of the colloidal composition of the slurry. The colloidal composition of the slurry can have a wide distribution of particle sizes, with diameters ranging from, for example, tens of nanometers (nm) to tens of micrometers (um). The concentration of particles may vary from, for example, xcx9c1012 particles per cubic centimeter (#/cc) for submicron particles to xcx9c1-100 #/cc for larger ( greater than 1 xcexcm) particles. The size distribution of particles in slurries typically can not be described by a single function but rather, often includes several independent modes. Monitoring particle size parameters is often desired for CMP applications because the size distribution of the small particles can determine the CMP performance. For example, large particles with diameters larger than a few micrometers can cause wafer damage.
Optical methods of detection and characterization have been used for non-intrusive, on-line monitoring of particle parameters in gas and liquid media. See, for example, U.S. Pat. No. 6,159,739, the disclosure of which is hereby incorporated by reference in its entirety. Optical methods include irradiation of the sample with light of known parameters (e.g. wavelength and intensity) and analysis of the scattered and/or transmitted light using different optical detectors.
Integral optical methods, also known as xe2x80x9censemblexe2x80x9d optical methods, allow in principle for the determination of the particle parameters of the scattering system as a whole. With these methods, particles of different sizes and compositions contribute simultaneously to the detected xe2x80x9csignal xe2x80x9d. Deconvolution of this composite xe2x80x9csignalxe2x80x9d, using an appropriate mathematical algorithm, is used to arrive at an estimate of the underlying particle size distribution. These integral optical methods usually involve several assumptions, such as the type (e.g., shape) of the size distribution, refractive index, and so forth.
Differential optical methods allow for the determination of the parameters of single particles. Accumulation of the information relating to the plurality of individual particles allows for the determination of the parameters of the system at whole. Because of the statistical nature of this process, the sensing volume is an important parameter for differential optical methods. The sensing volume is defined as that part of the sample suspension or dispersion, which is irradiated and from which the optical signal is collected. The larger the sensing volume, the greater the number of particles can typically be detected per given sampling period. At the same time, a larger sensing volume can generate larger scattering noise, which can ultimately determine the limit for particle detection. The optimal sensing volume can be determined by taking into account the degree of sample transparency, the concentration of smaller particles, the concentration of bigger particles, and so forth.
Application of optical methods for particle characterization of slurries and suspensions is often limited because of the high optical density of the sample, caused primarily by the high concentration of smaller particles. The high optical density of slurries and suspensions can cause multiple light scattering and a large blockage of light propagation through the sample. At these conditions it can be hard to obtain reliable and accurate information about the size distribution of the particles that scatter and/or block light.
The frequently wide variation in particle parameters of a liquid system can also create conflicting requirements. The sensing volume should be large enough to provide a statistically representative signal for the largest particles, having the lowest number concentration. However, the larger sensing volume can cause a higher optical density (turbidity) of the sample and more extensive multiple light scattering. In addition, a high concentration of abrasives in a slurry can damage optical cell surfaces that are in contact with the sample flow.
It would be desired to have the largest possible sensing volume, consistent with limitations imposed by the optical density and multiple light scattering of the sample. In addition, it would be desired to employ a differential method of optical detection to retain high resolution and sensitivity to the small number of relatively large particles that populate the particle size distribution and which largely define the quality of the slurry. Integral, or xe2x80x9censemblexe2x80x9d, methods of optical detection are significantly limited in their resolution and sensitivity in this respect.
U.S. Pat. No. 5,710,069 (Farkas et al.) discloses a method for measuring slurry particle size during substrate polishing, the disclosure of which is hereby incorporated by reference in its entirety. This method includes shining a light into a portion of the moving liquid-particle mixture having small numbers of relatively large particles and detecting and measuring the reflected light to determine the sizes of the particles. The signal includes a background due to scattered light from a relatively large, indeterminate number of particles. This scattered light is caused in part by the plurality of smaller particles that are present in a slurry at high concentrations.
Another known optical method which does not require slurry dilution, is described by Cerni and Sehler (Particle Optical Sensing for CMP Slurry, MICRO, 2000), and based on measuring the intensity of transmitted and scattered light of different wavelengths. This method involves data processing to reconstruct the underlying particle size distribution. It is an indirect method, which does not allow for the detection of single particles of larger size existing on a background of smaller particles at much higher concentration.
Another optical detection method is described in U.S. Pat. No. 5,818,583 by Sevick-Muraca et al. A system and method are disclosed for the self-calibrating, on-line determination of size distribution and volume fraction of a number of particles dispersed in a medium by detecting multiple scattered light from the particles. The multiple scattered light is re-emitted in response to exposure to a light source configured to provide light of time varying intensity at selected wavelengths. An estimation approach based on an expected shape of the size distribution and the mass of the particles is also disclosed.
U.S. Pat. No. 5,835,211 (Wells et al.) discloses an optical sensor for counting and sizing particles, the disclosure of which is hereby incorporated by reference in its entirety. This method includes measuring a light extinction (LE) and a light scattering (LS) signal representative of the particles. The light scattering and light extinction signals are combined to form a single composite signal, which increases in magnitude monotonically with an increase in the size of the particle passing through a beam of light. The combination of LS and LE signals allows an accurate measurement of particle size in both an upper range and lower range of particle sizes. The slurry is diluted for this monitoring technique to work properly, because only one particle should pass through the beam of light at a time. Additionally, a small sample of the slurry is first extracted and then substantially diluted before being analyzed. The analysis is therefore accomplished off-line, and therefore a delay can exist between the extraction of the slurry sample and the subsequent determination of the particle size distribution parameters.
A television system can be used to analyze an optical signal scattered by particles (Mavliev, R., 1992 Optical Determination of Size and Concentration of Particles below 100 nm: Method and Applications, In: Nucleation and Atmospheric Aerosols. Eds.: Fukuta, Wagner, A. Deepak Publishing, Hampton, VI, USA, 377-380). This approach divides the total sensing volume into approximately 103-104 smaller sub-volumes. The optical signal from each sub-volume is registered independently, resulting in a signal/noise ratio increase of approximately 1000 times. Detecting a signal from sub-volumes independently allows extending the range of measurable particulate concentrations. That is, the probability of coincidence of single particles can be reduced and the detectable concentration can be increased. This approach can be a partial solution for detection of signals from small particles of high concentration systems but is not known to be applied directly for slurries and emulsions with smaller concentrations.
Dilution of slurry samples, which is used by most optical methods, is not a desirable process because it can alter the particle size distribution parameters of the slurry, depending on the method and amount of dilution. It also can change the initial size distribution of the particles in a slurry (see Cerni and Sehler) because of the process of agglomeration or by introduction of foreign (contamination) particles associated with the fluid-used to dilute the initial concentrated slurry. Additionally, the concentration of the largest particles decreases substantially during extensive dilution, causing a reduction in statistics and therefore increased time to accumulate reliable particle size distribution information. Errors in the computed dilution coefficient directly affect the accuracy of the particle size distribution measurement relating to the original, concentrated sample.
In summary, dilution of the concentrated starting slurry sample is undesirable because particle size-parameters can, and often do, change; additional particles are usually introduced during the dilution process; and the concentration of the largest particles decreases significantly as a result of substantial dilution of the staring slurry sample.
Hydrodynamic focusing of a flowing fluid containing particles is used in scanning flow cytometry (Maltsev, 2000, Scanning flow cytometry for individual particle analysis, Review of Scientific Instruments, 71:243-255, the disclosure of which is hereby incorporated by reference in its entirety). Hydrodynamic focusing is based on delivering two concentric flows of fluid through a confined channel, or capillary. The outer xe2x80x9csheathxe2x80x9d flow is usually comprised of clean water (or other fluid), while the inner flow carries the particles to be measured. The inner flow diameter can be as small as 10 microns xcexcm. Using liquid flow cytometry, biological particles in suspension can be counted and classified in a rapid and reliable manner. The sample fluid containing the particles of interest is introduced into the focusing cell along with a transport liquid. The particles pass substantially one at a time through the flow cell, where they pass through an intense focused beam of light. The result is that the particles can be counted individually and analyzed, or measured, with regard to their xe2x80x9csignaturesxe2x80x9d of light scattering and fluorescence intensities. At present, liquid flow cytometry is employed for the analysis of white blood cells (leukocytes), and in the field of bacteriology. Examples can be found in U.S. Pat. Nos. 6,136,272 by Weigl et al.; in 6,109,119 by Jiang et al.; in 5,880,835 by Yamazaki et al.; and in 5,824,269 by Kosaka et al, the disclosures of which are hereby incorporated by reference in their entirety. Liquid flow cytometry methods have not been applied to concentrated slurry systems, because known methods do not take into account the typical high optical density of slurry systems caused by the existence of high concentrations of smaller particles.
Other documents of interest are as follows: U.S. Pat. No. 6,211,956 (Nicoli); 6,159,739 (Weigl et al.); 6,136,272 (Weigl et al.); 6,109,119 (Jiang et al.); 5,972,710 (Weigl et al.); 5,948,684 (Weigl et al.); 5,880,835 (Yamazaki et al.); 5,835,211 (Wells); 5,824,269 (Kosaka et al.); 5,818,583 (Sevic-Muraca et al.); 5,721,433 (Kosaka); 5,710,069 (Farkas et al.); 5,650,847 (Maltsey et al.); 5,690,895 (Matsumoto et al.) 5,663,503 (Kosaka); 5,548,395 (Kosaka); 5,159,403 (Kosaka); 5,007,732 (Ohki et al.); 4,983,038 (Ohki et al.); 5,739,902 (Gjelsnes et al.); 5,521,699 (Kosaka et al.) and 3,873,204 (Friedman et al.), the disclosures of which are hereby incorporated by reference in their entireties.
Similarly, the disclosures of the following documents are hereby incorporated by reference: Cerni and Sehler. Particle Optical Sensing for CMP Slurry, MICRO, 2000; J Adorjan, et al., Particle sizing in strongly turbid suspensions with the one-beam cross-correlation dynamic light-scattering technique. Applied Optics, 1999, 38:3409-3416; N. L. Swanson, B. D. Billard, and T. L. Gennaro, Limits of optical transmission measurements with application to particle sizing techniques. Applied Optics, 1999, 38:5887-5893; H. Schnablegger and O. Glatter, Sizing of colloidal particles with light scattering: correction for beginning multiple scattering. Applied Optics, 1995, 34:3489-3501; and P. Nefedov et al., Application of a forward-angle-scattering transmissometer for simultaneous measurements of particle size and number density in an optically dense medium. Applied Optics, 1998, 37:1682-1689.
Exemplary embodiments of the present invention are directed to a non-intrusive method for in-line or off-line monitoring of single particles over a wide range of sizes and concentrations, contained in systems comprised mostly of smaller particles. Exemplary methods can accommodate mixtures without requiring their dilution, and mixtures wherein the xe2x80x9ctailxe2x80x9d of largest particles in the particle size distribution can be accurately measured. Optical characterization of particles over a wide range of sizes and concentrations in concentrated systems is achieved using exemplary embodiments wherein the sample flow is made relatively transparent. The transparency of the sample flow can be arranged in the form of a xe2x80x9csheetxe2x80x9d flow, which is relatively thin in one dimension and wider in an orthogonal dimension. An exemplary optimal sample thickness can be determined using, for example, two criteria. The first criterion is based either on the absence of significant multiple scattering of light by the sample or the existence of relatively high sample transparency. The second criterion is based on the sensing volume necessary to obtain reliable statistical information on the large particles having a relatively low number concentration. For example, a sample fluid flow having a thickness in the range of approximately 5-500 xcexcm and width of 5-20 mm can be established.
Generally speaking, exemplary embodiments relate to a method, and associated apparatus, for detecting individual particles in a flowable sample, comprising the steps of: hydrodynamically focusing the sample, the sample being opaque to at least a first range of wavelengths of lightwaves; measuring transarency of the sample; compressing the sample to create a compressed sample which is transparent to at least one of the wavelengths of lightwaves; and identifying characteristics of individual particles contained in the compressed sample.
A sample fluid flow can be placed into operable communication (e.g., at least partially surrounded) with a flow of clean (i.e. relatively particle-free) transparent liquid (e.g., water) or other appropriate liquid. An optical cuvette containing a hydrodynamically focused flow of the sample fluid can be used to form a sheet flow of the sample fluid. The cuvette includes a converging part, a flat part and a sample introduction part. In the flat part of the cuvette the width of the flow channel is narrow in one dimension. For example, 0.1 mm xc2x110% or lesser or greater) and wide in the orthogonal dimension (for example, approximately 10 mmxc2x110% or lesser or greater). The flow channel parameters can be substantially constant (e.g., xc2x110% flow channel width in any direction) along the whole length of this flat part of the cuvette. The flat part of the cuvette is optically transparent and is used for characterization of the sample transparency and for the optical characterization of particles in the focused sample fluid.